[minor spoilers, none if you watched anime]
I'm not gonna lie- i used to hate Shingeki no Kyojin. When the anime first came out, i didn't really see anything exceptional in the anime- just another series about zombies with boring characters, ludicrous action, but this time with incredible soundtrack and animation. But outside of visuals, there really wasn't anything to it- you've seen first episode, you've seen all 25 of them.

But then, after 4 years 2nd season came out and.. i was intrigued, to say the least. One character revealed himself to be yet another humanity's enemy- the Titan. But instead of evil laugh and moustache-twirling (that you would expect from the level of story telling of the 1st season) he said something.. peculiar. That he's tired of being "soldier", that he had spent too much time with those idiots. And that he has to remember his duty of being a "warrior". What the hell does it all mean? i thought to myself. Is there something else to this manga than just giant zombies? Oh boy, how wrong was I.

Season 3 came around, more things were finally explained and pieces finally began to form a picture- first season of SnK was more of a lie, a prologue. We all were lured into thinking that Titans are this infinite danger, that all that matters is "glory to mankind".
But in reality the series started to develop a way deeper theme- the one fully explored only in the forth sea-- sorry, 3rd season, 2nd half.

And thus SnK changed from series about zombies into one about patriotism, xenophobia, racism, freedom at any cost. Characters i used to hate developed or the story developed to the point where you could finally see what Isayama intended them to be.

Take look at Eren- naive, loud, brave and bold shonen protagonist. Type of a character that would want to become king of pirates or a hokage. Incredibly dull and inadequate protagonist for a manga that tried o be a seinen.
But after the "basement" it became clear that Eren's naivety and childlishness was a trick designed by Isayama. Eren developed into this cold-hearted, cynic nihilist that wills to kill all his enemies, even if it may mean commiting war crimes.
But did anything really change about Eren? He's, in core, the very same character he was, since chapter 1. All that changed is the information about the world, which is amazingly depicted in the "shell" scene.
An ocean, that he thought to be a symbol of freedom, is yet another birdcage that keeps him safe from his real enemies. He thought titans were mindless monsters, but his "new" enemy isn't really that different from titans, they are maybe even more monstrous.

Other personality-lacking characters are also revealed to be actually brilliantly written. Reiner changes from your avarage bully into depressed paranoid, Berthold is clearly not doing anything he wants, Armin becomes more depressed... overall, this series changed from being edgy shounen into fully depressing experience.

What i think Isayama does the best is writing moments, not long-term stories. Any arc that doesn't rely of politics is just a blast to read, but the problems start when he wants to develop some "more real" intrigue- Uprising Arc is probably the worst in the series, since Isayama doesn't really know how politics work. He also likes using various shortcuts, where some of the more important plot beats are just there for the sake of simplifying the story. In short- some things "go too easy".
But, as i said, what Isayama does best are those *moments*. Full of emotions, surprising, sometimes even status quo changing. Till this day Reiner/Bertholt reveal scene is probably one of the best directed scenes i've seen in anime- kind of a one that you would watch 10 times, with your jaw dropping lower and lower.

The main reason i've written this review is the fact that majority of other reviews on MAL are from 5 years ago- where Snk was still about zombies, not philosophical conflict. And with series probably ending in half a year, i think it's the best time to maybe convince people who watched only 1st season to actually pick up.
Because trust me, it's totally worth it. I used to call SnK one of the most over-hyped series in existance, but now i'm fairly close to calling it a (flawed) masterpiece.